There is conventionally proposed a printer including a platen on which printing paper is rolled up, a rotary shaft disposed parallel to the platen, and a plurality of, e.g. 18 printing wheels having types along their circumferential margins and mounted on the rotary shaft, so as to effect a desired printing by controlling rotating amounts of the respective printing wheels. FIG. 16 shows a fragmentary view of such a prior art printer disclosed by Japanese Utility Model Publication No. 55-21798.
In FIG. 16, reference numeral 101 designates a rotary shaft, 102 denotes a printing wheel mounted on the rotary shaft 101. The printing wheel 102 has a bearing 102a receiving the rotary shaft 101, a group of types 102b along the circumferential margin thereof, and a selection ratchet 102c encircling the bearing 102a. Reference numeral 103 designates a drive claw which transmits the rotation of the rotary shaft 101 to the printing wheel 102. The drive claw 103 includes a shaft portion 103a inserted in a circular, recess 102e provided between the bearing 102a and selection ratchet 102c, a short and rigid first arm 103c having a claw portion 103b at the distal end thereof, and a long and resilient second arm 103d. The drive claw 103 is supported on the bearing 102a by the claw portion 103b inserted in a slit 102d of the bearing 102a. The claw portion 103b is engageable with a slit 101a provided in the rotary shaft 101 and defined by a radially oriented surface 101b and an opposed surface 101c angled with respect to the surface 101b. Reference numeral 104 denotes a selection claw engageable with the selection ratchet 102c and rotatable about an axle 105.
In this printer, when the rotary shaft 101 is rotated in arrow R direction while the claw portion 103b of the drive claw 103 engages the slit 101a of the rotary shaft 101, the rotation of the rotary shaft 101 is transmitted to the printing wheel 102 via the drive claw 103 so as to rotate the printing wheel 102. Also, the selection claw 104 pivots in arrow S direction and engages the selection ratchet 102c, so that the rotation of the selection ratchet 102c, i.e. the printing wheel 102 is stopped to place a desired type 102b at the printing position. The rotary shaft 101 continues its rotation for type selection of other printing wheels after the printing wheel 102 is held stationary. However, the rotation of the rotary shaft 101 is not transmitted because a confronting surface of the claw portion 103b of the drive claw 103 slides on the surface 101c of the slit 101a so as to expand the drive claw 103 against the resiliency of the second arm 103d.
The prior art printer, however, has some drawbacks to be improved as described below.
Transmission of the rotation from the rotary shaft 101 to the printing wheel 102 depends on a frictional force between the wall surface 101c of the slit 101a of the rotary shaft 101 and the opposed surface of the drive claw 103b. Therefore, the drive claw 103 is apt to disengage from the rotary shaft 101 due to a significant interia force caused by a sudden interruption of a high speed rotation or a rotation startup. Also, some kinds of external load to the printing wheel 102 would cause the drive claw 103 to disengage from the rotary shaft 101. To eliminate such a disengagement, it would be necessary to increase the resiliency of the second arm 103d of the drive claw 103. However, a large resiliency of the drive claw 103 and those of other drive claws associated with other printing wheels cause a significant increase in the torque for driving the rotary shaft 101 after one or more printing wheels are held stationary, and hence require a large-scaled motor for driving the rotary shaft 101, which increases the manufacturing cost as well as the running cost due to a large power consumption.
To overcome these drawbacks, the present Applicant proposed a printer shown in FIG. 17 and disclosed in Japanese patent application No. 60-4851. In FIG. 17, reference numeral 106 designates a selection ratchet mounted for relative and concurrent rotation with respect to the printing wheel 102, and 107 denotes a drive claw interposed between the printing wheel 102 and selection ratchet 106 to transmit the rotation of the rotary shaft 101 to the printing wheel 102. The drive claw 107 includes a hole 107a relatively rotatably receiving a shaft 102f provided on the printing wheel 102, a projection 107b engageable with a groove 101a of the rotary shaft 101, a rigid first arm 107c and a resilient second arm 107d. The selection ratchet 106 includes a series of teeth 106b along the circumferential margin thereof and a guide recess 106a receiving the distal end of the first arm 107a of the drive claw 107 to rotate the drive claw 107 about the shaft 102f. Reference numeral 108 designates a selection claw placed outward of and opposed to the selection ratchet 106. Other members corresponding to those of FIG. 16 are designated by the same reference numerals.
In this prior art printer, as shown in FIG. 17(a), when the rotary shaft 101 rotates in arrow 109 direction while the projection 107b of the drive claw 107 engages the groove 101a of the rotary shaft 101, the rotation of the rotary shaft 101 is transmitted to the printing wheel 102 via the drive claw 107 to rotate the printing wheel 102. Also as shown in FIG. 17(b), the selection claw 108 is driven by an electromagnetic clutch or other means (not shown) so that the distal end thereof engages the teeth 106b of the selection ratchet 106 to establish a unitary relationship with the selection ratchet 106. With a further rotation of the drive shaft 101, maintaining the unitary relationship with the selection ratchet 106, the distal end of the first arm 107c of the drive claw 107 moves outward along the guide recess 106a of the selection ratchet 106 to disengage the projection 107b of the first arm 107c from the slit 101a of the rotary shaft 101. Therefore, the printing wheel 102 looses the rotational force from the rotary shaft 101.
In this prior art printer, the selection ratchet 106 is a separate member from the printing wheel 102 and is provided with the guide recess 106a for pivotally engaging the drive claw 107, so that during type selection, the rotation of the rotary shaft 101 is transmitted to the printing wheel 102 via the surface intersecting the rotating direction of the rotary shaft 101 and with no substantial friction between the rotary shaft 101 and projection 107b. Therefore, the resiliency of the drive claw 107 can be rendered smallest also in high-speed rotation, and can decrease the torque for driving the rotary shaft 101. However, there still remains a problem in the prior art printer.
Since the printing wheel 102 and the separate selection ratchet 106 are relatively rotatably mounted, the printing, wheel 102 held stationary to place a selected type thereof at the printing position is apt to rotate slightly on possible application of an external force, e.g. a pivoting power of a hammer, which causes a positional error in the printed letters or numbers.